Cancer screening.

Especially in the emotionally charged field of cancer screening, which can have substantial public health implications for large numbers of healthy, asymptomatic people, it is important to achieve strong levels of evidence before promulgating new screening tools. This review of screening study methodology is intended to help the reader weigh such evidence and to evaluate reports which appear in the literature. It is an attempt to go beyond the often-stated intuition that early cancer detection finds cancers when they are easier to treat, at a time when survival is best. Examples tell us that sometimes this assumption has been true, sometimes not. A familiarity with the hidden biases in the supposition can be translated into everyday medical practice for screening tests in general. The practitioner can then match the strength of recommendation with the strength of the evidence behind the recommendation.